In a wide range of well and formation treatment methods it is desirable to use various materials such as bridging solids in a fluid loss pill for controlling downhole losses during operations or procedures, and then later to remove or destroy the materials, after they have fulfilled their function, to restore properties to the wellbore and/or subterranean formations such as permeability for oil and gas production, or to activate the materials or to fulfill a function such as a viscosity breaker or breaker aid.
Fluid loss control pills provide one example. When placing fluids in oilfield applications, fluid loss into the formation is a major concern. Fluid loss reduces the efficiency of the target subterranean formation and/or installed downhole completion equipment with respect to the potential to produce oil and gas. In addition time, fluid volume, and equipment can result in additional costs and compound the application. Thus, controlling fluid loss is highly desired. There are many oilfield applications in which filter cakes are needed during treatment of a wellbore. Filtercakes typically comprise an “external filter cake” and/or an “internal filter cake.” The filter cake prevents or lessens fluid leak off into porous rock at a desirable rate during a well treatment. In some cases the losses may become non detectable. Such treatments include drilling, completion, workover, stimulation, hydraulic fracturing or matrix dissolution, sand control, gravel packing, slurry-packing, frac-packing, and others. Typically, once well treatment has been completed, the continued presence of the filter cake is undesirable or unacceptable. Accordingly, the filter cake must be subsequently removed to reduce the extent of formation damage and facilitate production of hydrocarbons.
Numerous methodologies have been employed to control fluid loss during well treatment. Conventional water-based drilling and completion fluids, for example, often rely on biopolymers to provide viscosity and fluid loss control. The removal process may require acids, oxidizers and/or enzymes to degrade, disperse or dissolve polymer residue and filter cake buildup. It is, for example, a common practice for drilling fluid to use sized calcium carbonate as a bridging agent, in which case an acid plus a corrosion inhibitor package may be subsequently utilized for filter cake removal to either reduce skin damage or mitigate plugging of the selected sand control screen. The overall drilling/completion strategy is based, in part, on reducing risk with respect to time, required hardware, methodology, compatibility of fluids with the hardware and reservoir, and the health, safety and environmental (HS&E) concerns.
To prevent fluid loss during well treatment, solid bridging materials may be used. Bridging materials may be essentially insoluble, sparingly soluble, or slowly soluble in the well-bore fluid. Bridging materials may also have a particular shape and hardness such that they may be malleable, and/or round to non-spherical. These fluid loss additives are subsequently incorporated as filter cake components upon dehydration of the fluid loss pill when a differential pressure is applied on a porous medium. These include soluble or at least highly dispersed components of the fluids such as biopolymers and/or crosslinked polymers. Removal of the filter cake after well treatment is typically accomplished either by a mechanical means (scraping, jetting, and/or the like), and/or by subsequent addition of a breaking agent such as an acid, a base, an oxidizer, an ester or an enzyme, which may be added to dissolve at least a portion of the filter cake. The filter cake may also be removed by manipulation of the physical state of the filter cake by e.g., by emulsion inversion. Such removal methods usually require a tool or addition of another fluid, for example, to change the pH or to add a chemical, which may be accomplished in the wellbore. However, certain well treatments including hydraulic fracturing, gravel packing, and/or the like used to stimulate the production of hydrocarbons, water and other fluids from subterranean formations may not be suitable for such an addition of another component. Furthermore, methods which require fluid flow to remove the filter cake can result in slow to incomplete filter cake removal and may even result in a plugged stratum incapable of hydrocarbon production.
Other attempts used in the art include incorporating a breaker into the filter cake, which can provide a delay before activation of the breaker to remove the filter cake, thus allowing the completion or drilling phase to continue without catastophic losses of fluid. Examples of this methodology include, for example, esterification or encapsulation of the breaker. However, such technologies are often expensive and/or difficult to place and/or difficult to trigger.
Filter cakes may be formed from a pill that uses an oil, gelled with certain additives designed for the purpose. If the pill is water or brine, a gelled polymer e.g., a polysaccharide like guar may be used as the gelling agent. The polymer may be further crosslinked with a crosslinking agent, typically a metal ion from a boron, zirconium or titanium compound. Polymer-based pills tend to form a “filter cake” whereby they dehydrate or “coat-out” on the porous face of the wellbore. The process of filter cake formation is also called wallbuilding.
However, polymers have major deficiencies which include the filter cake being left in place, which can impede subsequent flow of hydrocarbons into the wellbore, and/or polymer or crosslinked polymer being left in the strata, which may impede or cut-off flow completely. Filter cake which is not removed may physically block the flow path into the wellbore, or may leave a high viscosity fluid in the strata which impedes the flow of hydrocarbons into the wellbore.
It is also known to treat a subterranean formation by pumping a colloidal suspension of small particles in a viscoelastic surfactant fluid system; see for example U.S. Pat. No. 7,081,439, which discloses a colloidal suspension and a viscoelastic surfactant which interact to form structures that effectively bridge and block pore throats. Colloidal suspensions are typically dispersions of discrete very small particles, spherical or elongated in shape, charged so that the repulsion between similarly charged particles stabilizes the dispersion. Disturbance of the charge balance, due for instance to removing water, changing the pH or adding salt or water-miscible organic solvent, causes the colloidal particles to aggregate, resulting in the formation of a gel. These particles are typically less than 1 micron in size, and typically in the range of from about 10 to about 100 nanometers. The dispersion is prepackaged as a liquid, transparent in the case of relatively low concentrations of particles, becoming opalescent or milky at higher concentrations. In any case, the dispersion may be handled as a liquid, which greatly simplifies the dosage.
It is also known to use a hydrolysable polyester material for use as a fluid loss additive for fluid loss control. After the treatment, the fluid loss additive degrades and so contributes little damage. Further, degradation products of such materials have been shown to cause delayed breaking of polymer-viscosified fracturing fluids. U.S. Pat. No. 4,715,967 discloses the use of polyglycolic acid (PGA) as a fluid loss additive to temporarily reduce the permeability of a formation. U.S. Pat. No. 6,509,301 describes the use of acid forming compounds such as PGA as delayed breakers of surfactant-based vesicle fluids, such as those formed from the zwitterionic material lecithin. The preferred pH of these materials is above 6.5, more preferably between 7.5 and 9.5.
Issues surrounding the removal of residual skin (e.g., removal of the filter cake) after well treatment are well recognized in the literature. Residual polymers, bridging solids, and other filter cake components hinder fluid flow and may totally block the well strata. Typical remedies include use of breakers, including encapsulated breakers that may require a specific or optimized breaker loading. The breaker may be added to the fluid/slurry and is intended to remove the filter cake, reduce the viscosity of the carrier fluid and generally facilitate clean-up after well treatment.
U.S. Pat. Nos. 4,848,467 and 4,961,466 is generally directed to the use of hydroxyacetic acid and similar condensation products which naturally degrade at reservoir temperature to release acid that may be a breaker for some polymers under some conditions and which offer fluid loss control. U.S. Pat. No. 3,960,736 is generally directed to the use of esters to provide a delayed acid, which will break the fluid by attacking both the polymer and the borate crosslinks. Similarly, acid generation mechanisms are employed in U.S. Pat. Nos. 4,387,769 and 4,526,695, which suggest using an ester polymer. U.S. Pat. No. 3,868,998 also mentions acid generation. These references rely on acid, which generally has a relatively low activity as the breaker. Other breakers include oxidative breakers, which are effective for removing polymers and other filter cake components. In addition, oxidative breakers may be used for breaking zirconium and/or titanium crosslinked gels, some of which are designed to be effective viscosifiers at low pH.
In addition, “breaker aids” may be used alone or in conjunction with breakers to promote breaker activity. Breaker aids known in the art include those disclosed in, e.g., U.S. Pat. No. 4,969,526, “Non-Interfering Breaker System for Delayed Crosslinked Fracturing Fluids at Low Temperature”, which is generally directed to using triethanolamine. See also U.S. Pat. No. 4,250,044. In addition, “retarding agents” or materials designed to inhibit cross-linking may be operable with both forming and cleanup of filter cake. See, e.g., U.S. Pat. No. 4,702,848, “Control of Crosslinking Reaction Rate Using Organozirconate Chelate Crosslinking Agent and Aldehyde Retarding Agent”, which is generally directed to using various aldehydes for this purpose. Breakers useful to remove filter cake may further include copper ion, silver ion, or the like, which are known to function as catalysts in conjunction with various chemical breakers, dissolved oxygen, or other oxidant source, in accelerating the breaker activity.
There is a need for improved methods of placing a fluid loss control agent and removing the fluid loss control agent to restore permeability to the producing formation, especially where removal of the fluid loss control agent does not require addition of another material to initiate removal of the filter cake. Similarly, there is a need for improved methods of breaking viscosified fluid loss control agent s, especially employing a breaker or breaker aid that does not require addition of another material to activate the breaker and/or breaker aid. There is also a need for improved methods employing a fluid loss control agent or breaker that can be placed downhole in insoluble form and subsequently removed by solubilizing the insoluble material, preferably without the need for an additional material.